1. Field of the Invention
The present invention relates to a method for the rapid starting of an asynchronous motor having a squirrel-cage rotor under load at an accelerated build-up of a motor torque through premagnetization.
2. Discussion of the Prior Art
A method of generally this type is known from the disclosure of U.S. Pat. No. 5,969,497 as a starting circuit for the rapid start (referred to as xe2x80x9cKick-Startxe2x80x9d) of an asynchronous induction motor which is operated from a single-phase AC-voltage network at a fixed voltage and fixed frequency in three-phase mode in a delta-connection, and is based therein on a controlled discharge of an additionally provided capacitive storage means for the direct-current premagnetization of at least one stator winding. The foregoing is; however, extremely expensive in the provision of the required circuitry. In any event, at the beginning of operation the capacitor has to be initially charged, which hinders the earliest possible employment of such a premagnetization. Moreover, the capacitor discharge into an inductive consumer with an iron core does not deliver any great differential quotient of the current cycle over time, and consequently does not provide a high induction effect on the rotor, so that just at the commencement of the rotor motion, this auxiliary starting measure provides only a relatively negligible contribution to the starting behavior of the motor, the magnetic time constant of which thus determines the time-delayed coming into effect of this auxiliary starting behavior. A further disadvantage in this respect is that, when the mains voltage is switched on at fixed frequency, the premagnetization is not field-oriented in relation to the rotating motor field. Such pre-magnetization which rotates in an unsynchronized manner therefore leads to a fluctuating torque precisely during the operationally critical start-up phase of the loaded asynchronous motor, and thus leads to an unstable operating behavior similar to that of asynchronous motor with a permanent-magnetic rotor, which has fallen out of step, that being harmful to the desired acceleration of the a synchronous motor in a manner which is extremely fast even under load; however, while being kinematically stable.
Accordingly, it is a primary object of the invention to be able to effect a defined and especially, an extremely fast acceleration of the asynchronous motor even under load from standstill to its rated speed of rotation within the magnetic rotor time constant.
In accordance with the present invention, the foregoing object is attained in that, through the intermediary of a program-controlled inverter, the stator has a magnetic field impressed thereon while the motor is still stopped, wherein the magnetic field changes over time and builds up an induction current in the rotor, whereupon a stator current is then immediately impressed and which possesses a spatial vector oriented approximately perpendicularly to a still quasi-stationarily occurring vector of the rotor current, so as to finally at the commencement of rotor motion cause the start of stator rotary field at an increasing frequency and amplitude.
In accordance with the foregoing, operation of the polyphase induction or asynchronous motor with short-circuited rotor in the form of a squirrel-cage rotor with an abruptly starting high start-up torque is no longer supplied from an AC-voltage network at fixed amplitude and supplied frequency, but in a manner which is known as such from a DC-voltage source by means of a controlled inverter with a pulse-modulated polyphase full-inverted bridge circuit for feeding the stator windings of the motor with a frequency and amplitude which can be varied in virtually any fashion. An example of such an inverter is described in the article xe2x80x98Digitale Steuerung eines Dreiphasen-lnduktionsmotorsxe2x80x99[xe2x80x98Digital Control of a Three-phase Induction Motor] by B. Maurice et al., in Design and Elektronik, Issue 8/92 of Apr. 7, 1992, pages 40 to 46.